"You've interviewed many, many people and I assumed that you would know the difference between a character and a real person"

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In this photo released by CBS, actor Joaquin Phoenix, left, gestures to host David Letterman on the set of the "Late Show with David Letterman," Wednesday, Sept. 22, 2010, in New York. This is Phoenix's first television appearance and first visit to the CBS broadcast since February 2009.

Updated at 4:47 AM PDT on Thursday, Sep 23, 2010

He’s still here – and sane.

A freshly-shaven, coiffed and lucid Joaquin Phoenix apologized to David Letterman Wednesday during his highly anticipated “Late Show” sit down, admitting that his 2009 disheveled appearance and oddball demeanor was all an act to promote his new mocumentary “I’m Still Here.”

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“You know I’ve always liked you,” Letterman said by way of getting down to business, citing his appreciation for Phoenix’s performance as Johnny Cash in “Walk the Line.” He then went right at the star over his strange behavior the last time he was on the show.

“And then a year and a half ago you come out and honestly it’s like you slipped and hit your head in the tub,” he said. " And I knew immediately when you sat down that something ain’t right. Because if you’re really the way you appeared to be, you don’t go out.”

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Phoenix, looking and sounding contrite, explained what was behind that surreal performance, which was revealed last week by the film's director, Casey Affleck, to be part of an elaborate ruse.

"You've interviewed many, many people and I assumed that you would know the difference between a character and a real person, so — but I apologize," Phoenix said during the CBS broadcast. "I hope I didn't offend you in any way."

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Letterman said he didn't take offense, adding that he relished the opportunity to dish out “dingers” like it was “batting practice.”

“We'd hoped to come on a talk show and I was looking for a beat down,” Phoenix said. “And I got one ... I want to thank you for that.”

Phoenix explained that he and co-conspirator Affleck wanted to do a movie that explored the idea of celebrity influenced by reality television shows. He explained that just because those shows use non-actors who appear under their actual names doesn't make them "real," adding that the shows were often scripted.

Letterman pointed out that the twice Oscar-nominated actor actually broke character at the end of the original segment. As Phoenix left the stage, he removed his glasses while thanking the talk show host, a move that reportedly angered director Affleck.

"I got in a lot of trouble with Casey about that," Phoenix said, adding that he was in a hurry to get off the set because of how uncomfortable the exchange had been.

Later in the interview, Letterman then said he wanted a million dollars for his appearance in the movie, saying that a snippet from the "Late Show" that made it into "I'm Still Here" did not constitute fair usage since the film wasn't actually a documentary.

"Can we talk about it privately?" Phoenix said.

"Yeah, we can go to one of your screenings," Letterman snapped back, making reference to the fact that "I'm Still Here" has been a box office bomb.

To dispel rumors that he was in on the hoax, Letterman pressed Phoenix to say the Feb. 11, 2009 appearance wasn’t scripted. The actor said that they were not working from a script and that Letterman had no prior knowledge of what was being planned.